1. Technical Field
The present invention relates generally to fuel control and, more particularly, to a method of measuring and compensating for a boil-off condition in a motor vehicle capable of operating on more than one type of fuel.
2. Discussion
Environmental and energy independence concerns have stimulated the development of alternative transportation fuels, such as alcohol fuels, for use in automobiles. Alcohol fuels include methanol and ethanol. A flexible fueled vehicle capable of operating on gasoline, or alcohol fuel, or any mixture of the two fuels, is therefore in demand. Modifications to the engine are necessary when operating on different fuels because of the different characteristics of each fuel. For example, an engine operating on ethanol or E85 (a blend of 85% ethanol and 15% gasoline) requires approximately 1.4 times the amount of fuel relative to gasoline at stoichiometry due to a lower energy content of the ethanol.
Air/fuel ratio in internal combustion engine design is typically considered to be the ratio of mass flow rate of air to mass flow rate of fuel inducted by an internal combustion engine to achieve conversion of the fuel into completely oxidized products. The chemically correct ratio corresponding to complete oxidation of the products is called stoichiometric. If the air/fuel ratio is less than stoichiometric, an engine is said to be operating rich, i.e., too much fuel is being burned in proportion to the amount of air to achieve perfect combustion. Likewise, if the air/fuel ratio is greater than stoichiometric, an engine is said to be operating lean, i.e., too much air is being burned in proportion to the amount of fuel to achieve perfect combustion. Alcohol fuels have a lower air/fuel ratio than gasoline at stoichiometric, so that the engine must be compensated for in the rich direction as the percentage of alcohol in the fuel increases.
In cold ambient temperatures, e.g., less than or equal to 35.degree. F., fuel requirements for engine start-up are increased dramatically over gasoline with the use of ethanol as a fuel. During vehicle start and run-up with an ethanol content from E40 (40% ethanol and 60% to gasoline) to E85 (85% ethanol and 15% gasoline), excess fuel may enter the crank case by traveling down the cylinder walls. The excess fuel commingles with the engine oil and is vaporized when the engine oil reaches the evaporation temperature for ethanol (approximately 170.degree. F.). The vaporized fuel then re-enters the engine induction system either through a PCV system or a PCV make-up system (i.e., crank case fresh air). Prior art systems directed toward compensating for methanol boil-off are ill-suited for ethanol fuel since ethanol boil-off corruption occurs almost immediately and does not slowly percolate as does methanol when engine oil reaches vaporization temperatures.
Therefore, it would be desirable to provide a method for measuring and compensating for excessive fuel vapor during boil-off corruption so that normal closed loop fueling adjustments may occur with minimal corruption, normal gasoline features may be unaffected by boil-off during blend changes from ethanol fuel to gasoline, and drivability may remain at target levels.